Ionizing radiation (IR) is an essential therapeutic modality for cancer, leukemia and lymphoma, but its usefulness is limited by its toxicity to normal tissue. Lymphoid and hematopoietic tissues exhibit a high degree of IR sensitivity and, therefore, typically determine the limits of IR therapy. Although DNA damage induced by IR is one of the main causes of tissue toxicity, an increasing body of evidence demonstrates that the expression of certain gene products via IR-induced activation of specific transcription factors also significantly contributes to this toxicity. Activation of particular transcription factors and the subsequent induction of specific gene products in lymphoid tissues and bone marrow (BM) may be responsible for their high radiosensitivity. We have recently demonstrated that IR induces lymphocyte-specific activation of NF-kappaB in vivo via a unique signal transduction pathway that is independent of IkappaB degradation but involves the production of reactive oxygen species (ROS) and protein tyrosine phosphorylation by protein tyrosine kinase(s) (PTK). It has been shown that activation of NF-kappaB by T cell receptor stimulation and genotoxic stress induces T cell expression of FasL which in turn, leads to apoptotic cell death. We hypothesize that a major component of IR-induced toxicity in lymphoid tissues and BM is mediated through the specific activation of NF-kappaB in lymphocytes via a lymphocyte-specific signal transduction pathway. This leads to the up-regulation of FasL expression and subsequent Fas-FasL interaction which causes undesirable lymphocyte apoptosis and bystander killing of hematopoietic stem cells (HSC) in the absence of IR-induced lethal DNA damage. To test this hypothesis, we will pursue the following Specific Aims: (a) to determine whether IR activates NF-kappaB in lymphocytes via tyrosine phosphorylation of IkappaBalpha by a lymphocyte-specific non-transmembrane PTK pathway; (b) to identify the cellular origin of IR-induced Fas and FasL expression in the spleen and BM and determine if Fas-FasL interaction plays a significant role in IR-induced lymphocyte and HSC apoptosis and (c) to examine whether activation of NF-kappaB mediates IR-induced T cell expression of FasL and determine if inhibition of NF-kappaB activation provides immune and hematopoietic protection via down- regulation of Fas-FasL interaction. These studies will provide new insights into the molecular mechanisms underlying the susceptibility of lymphoid and hematopoietic tissues to IR-induced tissue damage and offer novel strategies to improve tumor radiotherapy by reducing radiation toxicity.